A selective metabotropic glutamate receptor 7 agonist: Activation of receptor signaling via an allosteric site modulates stress parameters in vivo Kayo Mitsukawa, Rina Yamamoto, Silvio Ofner, Joachim Nozulak, Oliver Pescott, Snezana Lukic, Natacha Stoehr, Cedric Mombereau, Rainer Kuhn, Kevin H. McAllister, Herman van der Putten, John F. Cryan*, and Peter J. Flor† Neuroscience Research, Novartis Institutes for BioMedical Research, Novartis Pharma AG, CH-4002 Basel, Switzerland Edited by Edward M. Scolnick, The Broad Institute, Cambridge, MA, and approved October 25, 2005 (received for review September 16, 2005) Metabotropic glutamate receptor (mGluR) subtypes (mGluR1 to divided into three groups (I–III) according to sequence homol- mGluR8) act as important pre- and postsynaptic regulators of ogy, signal transduction mechanism, and agonist pharmacology neurotransmission in the CNS. These receptors consist of two (6, 7). Group I consists of mGluR1 and mGluR5, whereas domains, an extracellular region containing the orthosteric agonist mGluR2 and mGluR3 constitute group II. There are currently site and a transmembrane heptahelical domain involved in G four members within group III: mGluR6, a postsynaptic medi- protein activation and recognition of several recently synthesized ator of L-glutamatergic neurotransmission in retinal ON bipolar pharmacological modulators. The presynaptic receptor mGluR7 cells, and mGluR4, mGluR7, and mGluR8, which are presyn- shows the highest evolutionary conservation within the family, aptic receptors in the mammalian brain modulating release of but no selective pharmacological tool was known. Here we char- neurotransmitters such as GABA, L-glutamate, and, possibly, acterize an mGluR7-selective agonist, N,N؅-dibenzhydrylethane- monoamines (8). Pharmacological studies for group III mGluRs 1,2-diamine dihydrochloride (AMN082), which directly activates are most frequently performed with L-2-amino-4-phosphonobu- receptor signaling via an allosteric site in the transmembrane tyrate (L-AP4), an unselective agonist for all four receptor domain. At transfected mammalian cells expressing mGluR7, subtypes (9); recently, a subtype-selective positive modulator, AMN082 potently inhibits cAMP accumulation and stimulates activating mGluR4, has become available (10, 11). GTP␥S binding (EC50-values, 64–290 nM) with agonist efficacies mGluR7 is of particular interest given that studies in mice with comparable with those of L-2-amino-4-phosphonobutyrate (L-AP4) genetic ablation of mGluR7 indicate altered amygdala- and superior to those of L-glutamate. AMN082 (<10 ␮M) failed to dependent conditioned fear and aversion responses (12), and show appreciable activating or inhibitory effects at other mGluR reduced anxiety- and stress-related behaviors (13). Moreover, subtypes and selected ionotropic GluRs. Chimeric receptor studies mGluR7 ablation causes dysregulation of the hypothalamic– position the binding site of AMN082 in the transmembrane region pituitary–adrenal axis and increases hippocampal BDNF protein of mGluR7, and we demonstrate that this allosteric agonist has levels (14). Taken together, these data may imply a potential role little, if any, effect on the potency of orthosteric ligands. Here we for mGluR7 in the modulation of stress-related psychiatric provide evidence for full agonist activity mediated by the hepta- disorders. helical domain of family 3 G protein-coupled receptors (which have To date, selective pharmacological probing of mGluR7 acti- vation or inhibition in brain function was not possible because of mGluR-like structure) that may lead to drug development oppor- Ј tunities. Further, AMN082 is orally active, penetrates the blood– a lack of suitable tools. Now we have identified N,N - brain barrier, and elevates the plasma stress hormones corticoste- dibenzhydrylethane-1,2-diamine dihydrochloride (AMN082), rone and corticotropin in an mGluR7-dependent fashion. which acts as an allosteric agonist of mGluR7. The selectivity, Therefore, AMN082 is a valuable tool for unraveling the role of pharmacological mode of action, and presumable binding site of mGluR7 in stress-related CNS disorders. AMN082 are characterized extensively here, and we also report that the compound is orally active and brain-penetrable and that it modulates the release of the plasma stress hormones cortico- G protein ͉ mGluR sterone and corticotropin (ACTH) in an mGluR7-dependent fashion; i.e., it does not occur in mGluR7-deficient mice. protein-coupled receptors in vertebrates constitute a super- Gfamily of 1,000–2,000 transmembrane heptahelical proteins Materials and Methods that can be activated by a large number of extracellular signals Stable Cell Lines. Generation, culture, and pharmacological char- such as photons, hormones, neurotransmitters, and growth and acterization of stable cell lines for mGluR1b, mGluR2, mGluR3, development factors. These receptors transduce and amplify mGluR4, mGluR5a, mGluR6, mGluR7a, mGluR7b, mGluR8a, cellular signals by the activation of G proteins, which in turn N-methyl-D-aspartate receptor (NMDAR) 1a͞2A, NMDAR1a͞ modulates cytoplasmic second-messenger and ion levels (1). 2B, and GluR3i have been described (10, 15–18). Sequence comparison among the different G protein-coupled receptors revealed the existence of at least six receptor families (2). One of them, family 3, comprises receptors for extracellular Conflict of interest statement: The authors are employees of Novartis Pharma AG, which is calcium, pheromones, GABA, and L-glutamate. This receptor interested in developing glutamatergic compounds for stress-related disorders. family is characterized by a large N-terminal extracellular do- This paper was submitted directly (Track II) to the PNAS office. main that has been demonstrated by site-directed mutagenesis Abbreviations: mGluR, metabotropic glutamate receptor; L-AP4, L-2-amino-4-phospho- Ј and x-ray crystallography to contain the binding site for ortho- nobutyrate; AMN082, N,N -dibenzhydrylethane-1,2-diamine dihydrochloride; ACTH, cor- ticotropin; NMDAR, N-methyl-D-aspartate receptor; L-SOP, L-serine-O-phosphate. steric agonists (3–5). Metabotropic glutamate receptors *Present address: Department of Pharmacology and Therapeutics, School of Pharmacy, (mGluRs) are members of family 3 and are activated by the University College Cork, Cork, Ireland. major excitatory neurotransmitter of the mammalian brain, †To whom correspondence should be addressed at: Neuroscience Research, Novartis Insti- L-glutamate. The mGluRs act as important pre- and postsynaptic tutes for BioMedical Research, Novartis Pharma AG, WKL-125.6.08, CH-4002 Basel, Swit- regulators of neurotransmission in the CNS, and there are at zerland. E-mail: peter࿝josef.fl[email protected]. least eight subtypes (mGluR1 to mGluR8), which have been © 2005 by The National Academy of Sciences of the USA 18712–18717 ͉ PNAS ͉ December 20, 2005 ͉ vol. 102 ͉ no. 51 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0508063102 Downloaded by guest on September 28, 2021 Construction of Chimeric Receptors and Corresponding Cell Lines. cDNAs encoding wild-type mGluR6 and mGluR7b were de- scribed previously (16, 19). cDNAs encoding chimeric mGluR6͞7b and mGluR7͞6 proteins were constructed by using the PCR overlap extension approach (3): the mGluR6͞7b con- struct contains 570 aa derived from the N-terminal extracellular region of mGluR6 and the remaining C-terminal portion of mGluR7b, comprising the entire transmembrane region; mGluR7͞6 is essentially the reverse chimera with the fusion point at amino acid 575. The four constructs were used to generate stably transfected cell lines by means of Flp recombi- nase-mediated integration (kit from Invitrogen) in CHO-C4 cells (Novartis Pharma AG, Basel), which contain one specific Flp recognition target (FRT) site; the identical host was used for all of the four new cell lines. GTP␥35S Binding Assays. Membranes were prepared from trans- fected mGluR2-, mGluR3-, mGluR4-, mGluR6-, mGluR7a-, mGluR7b-, mGluR8a-, mGluR6͞7b-, and mGluR7͞6-express- ing cells, and GTP␥35S binding assays were conducted by using the protocol described by Maj et al. (10). Second-Messenger Assays. Measurements of cAMP accumulation were performed as previously described by using CHO cell lines stably expressing individual mGluR subtypes (15, 16). Measure- Fig. 1. AMN082 activates G protein signaling via mGluR7. (A) Effect of ␮ ment of [3H]inositol phosphate formation was done according to DL-AP4 (4 mM, dotted line) and AMN082 (0.01–3 M, solid line) on the Gasparini et al. (18), and calcium measurements were done as inhibition of forskolin-stimulated cAMP accumulation in CHO cells stably expressing mGluR7b. (B) Effect of L-glutamate (L-Glu) and AMN082 on CHO described by Maj et al. (10). PHARMACOLOGY mGluR2 cells. The results are normalized to control (30 ␮M forskolin- stimulated cAMP levels) and were pooled from at least six measurements 3 [ H]LY341495 Binding Assay. Membrane fractions of CHO cells obtained in three independent experiments and expressed as means with stably expressing mGluR7a (see above) were diluted in assay SEM. **, P Ͻ 0.01 vs. control; Dunnett’s t test. (C) GTP␥35S binding experiments buffer (10 mM KH2PO4͞100 mM KBr, pH 7.6), homogenized using membranes from CHO mGluR7b cells. The indicated concentrations of briefly by using a Polytron homogenizer (IKA Labortechnik, AMN082, DL-AP4, and L-glutamate were applied alone and in combination Staufen, Germany), and incubated for 10 min at 30°C. Assay with each other. Three independent